Xanthine oxidase (XO)-catalyzed generation of superoxide anion is thought to contribute to the pathogenesis of autoimmune arthritis. The relative importance of XO to the pathogenesis of arthritis is demonstrated in numerous rodent models of autoimmune arthritis in which allopurinol, a prototypic XO inhibitor, profoundly reduces joint injury. Accordingly, XO inhibitors have been proposed as a novel class of therapeutics for clinical rheumatoid arthritis (RA). Currently marketed XO inhibitors, allopurinol and oxypurinol, are however considered poor candidates for therapy of clinical RA, as they are weak (IC50 = 40 micromolar) and elicit hypersensitivity reactions in 10% of the population. To address this opportunity, we are developing a series of potent novel XO inhibitors "AN series" (IC50 = 40-200 nM). In a pilot study, we have obtained evidence that enteral administration of a lead AN agent (AN-01-24) reduces the incidence of experimental joint inflammation by 70%. In other regional models of inflammation, our clinical development XO inhibitor, AN-260, has been shown to abolish histologic injury in a DSS model of colitis and to prevent neutrophil infiltration and pro-inflammatory gene expression in an LPS model of ARDS. AN-260 is currently completing formal pre-clinical GLP and GMP studies, in preparation for Phase I clinical testing in 6 months. In order to establish the feasibility of this technology as a potential novel treatment of RA, we now propose to compare the efficacy of AN-260 with the currently marketed XO inhibitor, allopurinol, in two classic rodent models of autoimmune arthritis: a murine model of collagen-induced arthritis and a rat model of adjuvant arthritis. In order to benchmark the efficacy of XO inhibition in these model systems, we will test in parallel the effect of a traditional anti-oxidant, N-acetyl-L-cysteine. We expect that AN-260 will dose-dependently ameliorate gross and histologic injury, preserve joint function, diminish malondialdehyde formation (a marker of lipid peroxidation), and decrease myeloperoxidase activity (a marker of neutrophil infiltration). Based on preliminary data in other inflammatory models, we expect that AN-260 will be more effective than allopurinol and N-acetyl-L-cysteine. In a Phase 2 SBIR, we will carry out a Phase IIa clinical investigation to establish the efficacy of AN-260 in reversing symptomatic clinical RA.